The Senate’s landmark effort to overhaul U.S. immigration laws faces its first pivotal challenge beginning Thursday, when it begins to consider a raft of amendments that could threaten the fragile compromise struck by a bipartisan group of negotiators.

When the Senate Judiciary Committee kicks off what is expected to be a lengthy process of debating and amending the legislation, its members will bring competing goals to the tussle. The architects of the bill, four of whom sit on the committee, will try to shepherd the bill to the floor with its core intact, while still giving colleagues a fair shot at improving the product. In contrast, opponents of immigration reform aim to sap its momentum by slowing the process and buying time for a backlash to build.

Republican Senator John McCain of Arizona, a member of the eight-member bipartisan group which crafted the bill, said he was “guardedly optimistic” about its prospects, but warned that substantial changes could torpedo its chances. “If there are efforts made to destroy that delicate compromise,” McCain said, “then it could fall apart.”

In their bid to sink the bill, detractors will try to drown it under a deluge of amendments. By the 5 p.m. deadline on Tuesday, the 18 members of the Judiciary Committee — 10 Democrats, 8 Republicans — had filed some 300 amendments. Some are modest tweaks designed to patch holes or tinker around the edges. Many others are less helpful.

Nearly two-thirds of the amendments were filed by Republicans. A majority of those were authored by two vocal critics: ranking member Charles Grassley of Iowa, who submitted 77 amendments, and Alabama’s Jeff Sessions, who tacked on 49. Ted Cruz of Texas proposed blocking a path to citizenship for anyone who has ever been willfully in the U.S. without legal status — an amendment that would strip out the heart of the bill. Sessions wants to limit the total number of worker visas. Orrin Hatch would require immigrants to submit DNA samples to authorities. His Utah colleague Mike Lee would allow unauthorized immigrants to work only as domestic servants, in roles such as cooks, maids, butlers, babysitters, janitors or “footmen.”

Opponents like Sessions and Grassley have two strategies, says Frank Sharry, executive director of the pro-immigration reform group America’s Voice. The first is to slow the process to a crawl. If Judiciary Committee Chairman Patrick Leahy tries to accelerate it, Republicans “will claim that Democrats are shutting down Republican votes just like Obamacare. They’ll say the process wasn’t fair, and hope a backlash from the grassroots will materialize,” Sharry says. “The second thing is to frame amendments that they hope will make the proponents of the bill look bad, and fire up talk radio.”

Grassley argues a painstaking process is necessary to ensure a good bill. “This debate should be thoughtful and thorough,” he said in a statement. “It will be arduous and it ought to be deliberate.” The full bill was filed weeks ago, but the Iowan is already griping that it should be stalled until Homeland Security Secretary Janet Napolitano answers questions posed at a hearing that focused on the Boston Marathon bombing. “How can we markup bill Thurs wo [sic] answers,” he tweeted this week.

To some extent, the threat of death-by-amendment comes from the left as well as the right. Leahy has filed an amendment that would extend protections for same-sex couples by allowing gay citizens to sponsor their foreign partners or spouses for green cards. While gay-rights groups have lobbied Democrats to include same-sex marriage provisions in the bill, negotiators declined to, fearing that doing so would endanger Republican support. Each GOP author of the legislation opposes the amendment, and several have said its inclusion would kill the bill.

If Leahy offers the amendment in committee, it would likely pass, since a simple majority is all that’s required. (On the Senate floor, he would face the 60-vote threshold that has become routine thanks to the constant threat of a filibuster.) Leahy could opt not to offer an amendment that could become a poison pill, but he has argued it wouldn’t tank the bill, and “it seems like he’s committed to offering it in committee,” Sharry says. “Would it pass? It seems very likely to. The question becomes: are Republicans bluffing, or are they serious that they’ll walk?”

Leahy has given some clues about how he will sequence the committee’s work, which he hopes will wrap up by the end of the month before heading to the full Senate in June. First the committee will address the technical glitches in the legislation, followed by consideration of its “triggers” — the security standards that have to be met before immigrants can embark on the path to legal status and then citizenship. Next come amendments that focus on border security, followed by immigrant visas and enforcement. If they want to preserve the bill’s bipartisan backing and win over converts, Democrats can’t swat away all Republican amendments. Gang of Eight negotiators have said their goal is to garner 70 votes, or a majority of both parties in the upper chambers, in a show of force that would put political pressure on the Republican-controlled House.

While the blizzard of amendments could be unwieldy enough to gum up the calendar, controversial bills often draw a high volume of proposed tweaks. “It’s not unheard of,” says Sarah Binder, an expert on congressional procedure at George Washington University and the Brookings Institution. “Any major legislation like this, particularly when there are some pockets of opposition, is going to attract a lot of amendments.”

A simple majority of committee members is all that’s required to send the bill to the full Senate, which means a party line vote would be enough. Barring dramatic modifications, the bill is also likely to attract the support in committee of Arizona’s Jeff Flake and South Carolina’s Lindsey Graham, two Republican members of the Gang of Eight. Observers say the margin in committee is likely to be 12 to 6 or 13 to 5. And yet, if the avalanche of amendments prompts Democrats to blow through them and kindles a grassroots backlash, the bill could head to the floor bruised if not broken.